1. Field of the Invention
This invention relates to information storage and, in particular, to optical information storage.
2. Art Background
A variety of systems has been proposed and employed for the optical storage of data. An example of these systems is the use of tellurium alloys that are deposited on a substrate. To store information in such a medium, a localized region of the tellurium alloy is melted. By melting and/or ablating the tellurium alloy in a localized area, a void is produced that exposes the underlying substrate. Two typical storage medium configurations are employed to produce an optical change upon void formation. In the first configuration, a second substrate is affixed below and spaced from a tellurium coated, transparent substrate so that an air space is provided between the two substrates. Since the tellurium alloy has a smooth surface, reflection occurs before a void is formed. After void formation, light traverses the first transparent substrate and is internally reflected in the air space until it is absorbed. Thus, the untreated surface appears reflective, and the voids appear black.
In a second configuration, the tellurium alloy is deposited on a transparent dielectric which in turn overlies a metallized substrate. The thickness of the dielectric is carefully chosen to produce destructive interference of the incident light used for writing the stored information. Thus, before writing, the medium absorbs at the wavelength of the reading light source. Where voids are formed, the absence of the tellurium alloy precludes destructive interference of the reading light. Light is reflected from the exposed, underlying metallized surface and is thus observed. Therefore, in either configuration, digital information is represented by optically absorbing areas and optically reflective areas. By using a laser to write such information, it is possible in a tellurium based medium to store data with a storage density on the order of 30 megabits per square centimeter. (Storage density is the number of bits stored per unit surface area of the storage medium.)
Despite extensive investigation, there is still a number of considerations which make a tellurium based medium somewhat undesirable. In all configurations, the tellurium alloys employed are relatively unstable. Tellurium readily oxidizes in the atmosphere to form transparent oxides of tellurium. Obviously, the transparent tellurium oxide is no longer suitable for optical storage. To prevent extremely rapid oxidative degradation of the tellurium medium, it is necessary to employ a hermetic sealant. This sealant substantially increases the cost of the storage medium. Further, in extreme oxidation conditions such as in fires, toxic tellurium compounds are formed and present a potential health hazard. In this regard, the use of tellurium in the fabrication of such media also requires suitable precaution to insulate workers from tellurium exposure.
Additionally, the contrast between the absorbing tellurium alloy and a void, although acceptable, leads to a certain degree of error upon reading the stored information by using a light source, such as a laser, and a detector, such as a silicon photodiode, to discern these optical differences. (Contrast is the ratio of the optical reflectance of the initial medium to the optical reflectance of the written medium or the inverse of this ratio, whichever is larger.) Ideally, for relatively accurate retrieval, contrasts greater than 5 to 1 are desirable. By various expedients, e.g., by increasing the tellurium alloy thickness, tellurium based storage media have been made with contrasts typically up to 15 to 1. However, the use of these expedients generally substantially increases the energy needed to write information. Although the contrast and sensitivities of tellurium are adequate to produce useful storage media, these contrast levels, together with the relative instability of the medium, are not entirely desirable.